An electrochemical cell, in its simplest terms, consists of an anode (the oxidizing electrode), a cathode (the reducing electrode) and an electrolyte. In order for the electrochemical cell to function, the electrolyte must be compatible with the mechanisms of oxidation and reduction at the electrodes. As well, it must provide a conductive path for the transport of ionic species between the electrodes.
The electrochemical cell concept is broadly applied in industrial and scientific operations. Electrolytic cells are used in electroplating, water purification, and the production of high purity gases and metals while electrochemical cells, such as batteries and fuel cells provide a convenient means of energy storage and generation.
Also, due to their very high level of sensitivity, electrochemical cells are used for measurement in a variety of analytical procedures and many laboratory and process control instruments depend on the electrochemical cell as the sensing element for their function.
U.S. Pat. No. 4,960,497 discloses a system wherein an electrolytic cell measures oxygen in the ppb range. In this system, the dissolved oxygen in the electrolyte is removed to allow for an accurate reading of the oxygen concentration in a gas sample. However, in this system, when measuring in the 0-100 ppb range, it was found that in some instances the signal-to-noise ratio was not high enough to provide a consistently accurate reading.
There are other electrochemical systems currently available which measure oxygen in the 0-100 ppb range. In these systems, the anode, typically lead or cadmium, functions as a consumable half cell. A drawback inherent in these systems is that because the anode is consumed, its properties or characteristics change over time and this can affect the accuracy of the measurements, particularly in the ppb range. This drawback is typical of the consumable (battery) type electrochemical oxygen sensor.
The present invention is directed to an electrochemical system and a method for measuring an analyte, i.e. oxygen, hydrogen, ammonia and hydrazine in the ppb range. Broadly the system functions as a hydrogen-oxygen alkaline fuel cell configured to generate a current which is linear to the rate at which the analyte is either reduced at a cathode, i.e. oxygen or oxidized at the anode, i.e. hydrogen, ammonia, hydrazine. The inventive system differs from prior art fuel cells in that prior art fuel cells are designed or configured to optimize the output of electrical energy and are not designed to function as an analyte sensor, particularly in the ppb range. The present invention differs from the prior art sensor ppb oxygen systems described above because in the inventive system the anode is non-depleting or non-consumable. In this inventive system, no chemical changes take place within the sensor, thus allowing the oxygen measurement to remain stable over time.
In the preferred embodiment, the invention comprises an electrochemical cell. A gaseous stream containing the oxygen to be measured contacts a cathode catalytically optimized for oxygen. The oxygen is reduced forming hydroxyl ions. Simultaneously, a stream of hydrogen contacts a non-depleting anode. The hydrogen is oxidized (in the presence of the hydroxyl ions), and collectively these reactions generate a current which is proportional to the rate at which oxygen is reduced at the cathode. The current measured corresponds exactly to the changing concentration of oxygen in the gaseous stream.
One advantage of this invention is that the system is a relatively clean system, the redox byproduct being water as opposed to lead oxide (lead anode) or cadmium hydroxide (cadmium anode). In this system, the water byproduct harmlessly evaporates, while the lead oxide and cadmium hydroxide byproducts build up in the sensor cell and act as inhibitors to the electrochemical reactions.
Distinct advantages over the previous ppb oxygen sensor system disclosed in U.S. Pat. No. 4,960,497 include higher oxygen sensitivity, lower background offset, less offset drift, improved linear response, improved speed of response and reduced temperature sensitivity.